In this article we search to bring to light one of the unknown aspects of the work of one of the 20th century's greatest psychologists: Bärbel Inhelder. Thus far recognized only as the most remarkable Piaget collaborators, she has been ignored as the creator of a scientific method apt to refute (instead of corroborating) a hypothetical-deductive model in the realm of Psychology, prior or simultaneously to Popper's idea. Based on Piaget's mathematical model, the grouping, she did her study on oligophrenics. She demonstrates that the more severe the brain lesions, the lesser the presence of the groupings' structural sketches underlying the children's actions and speech, and vice-versa.